FIG. 4 depicts a startup circuit used to detect brown in brown out in a rectified AC signal. The circuit includes a normally on transistor (e.g., a JFET) connected in series to a Zener diode ZD1. A Zener diode allows current to flow in the forward direction in the same manner as a conventional diode, but also permits it to flow in the reverse direction when the voltage is above a certain value, the so-called Zener voltage.
The startup circuit depicted in FIG. 4 is used to detect an input voltage at a drain to thereby generate a detection signal. In this voltage detector, a Zener diode ZD1 has a cathode connected to a terminal that receives the input voltage, a junction field effect transistor (JFET) has a drain acting as an input terminal connected to an anode of the Zener diode ZD1, a source acting as an output terminal and a gate acting as a control terminal, a resistor Rgs is connected between the source and the gate of the JFET, and a switch M2 is connected between the gate of the JFET and ground, and is controlled by a control signal from a JFET control block. When the switch M2 is off, the voltage detector does not detect the input voltage at the input terminal. When the switch M2 is on, if the input voltage at the detect terminal is higher than the breakdown voltage of the Zener diode ZD1, the JFET operates and thereby produces a current ID flowing through the JFET from drain to source. Since the switch is on, the gate of the JFET is grounded, and the current ID will flow through the resistor Rgs to increase the source-gate voltage of the JFET, i.e., the gate-source voltage of the JFET decreases. The gate-source voltage VGS of the JFET will become equal to a pinch-off voltage of the JFET, and thus the voltage drop across the resistor Rgs will be equal to the pinch off voltage.
When the switch M2 is on, if the input voltage is lower than the breakdown voltage of the Zener diode, there will be no current ID flowing through the JFET, and since the gate of the JFET is grounded, the JFET has its source and gate at the same potential, i.e., the voltage drop across the resistor Rgs is zero. Because the leakage current of the JFET is almost zero, the voltage detector has nearly no power loss when detecting the input voltage Vin.
As discussed above, according to the voltage drop across the resistor Rgs, the voltage detector can determine whether the input voltage Vin is higher or lower than the breakdown voltage of the zener diode ZD1, so that a detection signal can be derived from the voltage drop across the resistor Rgs. If the JFET is grounded by the switch M2, the source voltage of the JFET can be directly taken as the detection signal. In this voltage detector, the breakdown voltage of the Zener diode ZD1 is taken as a reference voltage for detection, which may be altered by selecting or adjusting the breakdown voltage of the zener diode ZD1 or by adding more zener diodes connected in series.
It is within this context that embodiments of the present invention arise.